Prism for high contrast projection

ABSTRACT

Prism elements having TIR surfaces placed in close proximity to the active area of a SLM device  302  to separate unwanted off-state  324  and/or flat-state  326  light from the projection ON-light bundle  322 . The TIR critical angle of these prisms is selected to affect either the off-state light or additionally, any portion of flat-state light reflected from the SLM. These TIR surfaces are placed to immediately reflect the unwanted light as it comes off the SLM, thereby preventing the contamination of light along the projection path, which tends to degrade the system contrast. To further improve the optical performance of the system, these TIR prisms can be attached directly to the SLM package  300 , completely eliminating the package window. Also, these TIR prism can be coupled to DMDs having asymmetric mirrors, which tilt through a larger angle for the ON-light to provide high etendue and lumen output, but through near-zero degrees for the OFF-light, thereby improving the separation of any unwanted light from the off/flat states.

THIS APPLN CLAIMS BENEFIT OF 60/345,732 filed Dec. 31 2001.

FIELD OF THE INVENTION

The present invention relates to optical elements used in projectionsystems and more particularly to high contrast prisms, which separateunwanted light reflected from the OFF pixels from the projectionillumination bundle reflected from the ON pixels.

BACKGROUND OF THE INVENTION

In a typical spatial light modulator (SLM) projection system, such as adigital micromirror device (DMD) based system, undesired off-state andflat-state light can overlap the desired image projection illuminationfor some distance along the optical path and is often reflected on tothe screen causing a reduction in image contrast. In high powerprojectors, the off-state light can be of sufficient duration andmagnitude to increase the thermal energy in the optics and otherhardware, resulting in optical distortion, mechanical stress, and/orimage misconvergence.

This unwanted light can result from scattering of light off varioussurfaces, such as the device package, device structure, window, andprisms. Current approaches to handle this unwanted light often usebaffles or bounded apertures in the projection light path. However,bounded apertures that pass the projection light also pass any off-stateand flat-state light that spatially and angularly overlap the clearaperture region. Other approaches direct the unwanted light into anoptical heat sink (light trap), often reflecting off various totalinternal reflective (TIR) surfaces along the optical path, but do thistoo far along the optical path to prevent contamination of the desiredprojection light.

FIGS. 1 a and 1 b are diagrams illustrating the operation of a typicalDMD light modulator. These devices are constructed on a siliconsubstrate 100, which contains an underlying memory structure used tocontrol the binary state of each micromirror. The micromirrorsuperstructure is suspended by means of torsion hinges, supported byposts 102, on top of the substrate. The superstructure consists of ayoke 104, which is attached to the torsion hinges, and a highlyreflective metal micromirror 106 attached to the yoke by a mirror post.The structure is caused to tilt about the diagonal hinge axis due toelectrostatic forces created by an electric field established betweenaddress pads connected to the memory structure and the yoke/mirror biasvoltage. The yoke/mirror structure typically rotates from +10° 108 forON pixels to −10° 110 for OFF pixels.

In operation, as shown in FIG. 1 c, incoming light 118 typically entersthe system at two times the tilt angle (20°), such that light strikingmirrors rotated +10° 112 (ON pixels) is reflected along an ON projectionpath 120 through a projection lens on to a display screen. On the otherhand, incoming light 118 that strikes mirrors rotated −10° 114 (OFFpixels) is reflected along a second off-light path 122. Also, some ofthe incoming light strikes various flat surfaces and edges in and aroundthe DMD package and is reflected 126 off the device 116 as additionalunwanted light. Where adjacent mirrors are in the ON and OFF states,respectively, it is possible for some light to pass through the gapbetween the mirrors, getting underneath the mirror and bouncing around124, finally exiting with some of the light 128 finding its way into theprojection light path, thereby reducing the contrast of the projectedimage. This condition exposes itself with the background of the imagebeing lighter than desired.

Previous solutions have attempted to improve the DMD contrast withabsorptive coatings under the DMD micromirrors. Apertures have also beenincorporated to filter unwanted light from the DMD and the projectionpath, but do not provide adequate filtration, especially for lightreflecting from the underside of the off-state mirrors.

The use of a TIR surface to filter the off-state light just prior toentry into the projection lens has been tried, but this is too far alongthe optical path to separate out the unwanted light, and this approachdoes not address the dependency between early filtration of theoff-state light and the opportunity that exists to reduce the DMDoff-light tilt angle.

What is needed to improve the contrast in these projection systems is toseparate the unwanted flat-state and off-state light from the projectionlight bundle immediately after the light is reflected off the surface ofthe DMD. The present invention discloses multiple embodiments foraccomplishing this unwanted light separation. By controlling anddirecting the unwanted light immediately to a light absorbing heat sink,the projection light remains free of these offensive light rays, and asa result can be optically and geometrically optimized for imageprojection to the screen. Also, to further improve the etendue and lumenoutput of a projection system, an asymmetric DMD having micromirrorsthat tilt +x degrees (typically +10°) in the ON direction, but less thanx degrees (typically 0 to −4°) in the OFF direction and coupled tooptical prisms having OFF light TIR surfaces in close proximity to thelight modulators, is disclosed. This approach provides a very fast DMDbased projection system that optically switches the unwanted light intoa heat sink at a predetermined threshold value. A DMD having a largerON-mirror tilt angle and a near-flat OFF-mirror tilt angle coupled tothe optics of the present invention, having TIR surfaces to direct theunwanted off-light immediately away from the projection light bundle canprovide an optimal solution for improving the contrast in projectionsystems. Finally, in order to provide a low-cost solution to theunwanted light separation problem, a single element prism embodiment isalso disclosed in the present invention.

SUMMARY OF THE INVENTION

This invention discloses prism elements having TIR surfaces placed inclose proximity to the active area of a SLM to separate the projectionlight bundle from off-state or flat-state light, or both. Embodimentsfor both single-SLM and multiple-SLM projection systems are disclosed,which provide high contrast projection solutions. The critical angle ofthe TIR surface in these prisms is selected to affect either theoff-state light or additionally, any portion of flat-state lightreflected from the SLM. These TIR surfaces are placed so as toimmediately reflect this unwanted light as it comes off the SLM, therebypreventing the contamination of light along the projection path, whichtends to degrade the system contrast. For low-cost projection systems, asingle element TIR prism having two TIR surfaces is also disclosed.

Furthermore, the TIR prisms can be attached directly to the SLM package,completely eliminating the package window. This not only allows for theunwanted light to be removed from the system sooner, but also othersystem contamination such as dust and moisture is reduced and the numberof optical elements that the light has to pass through, as well as thesystem size and cost are reduced.

Finally, an embodiment where the TIR prisms are coupled to DMD(s) havingasymmetric mirrors, which tilt through a larger angle for the ON lightto provide high etendue and lumen output, but through a near-zero degreeangle for the OFF light is disclosed, thereby improving the separationof any unwanted light from the off and/or flat state light. In thisapproach, as the DMD mirrors transition from ON to OFF or OFF to ONstates, the unwanted light switches to the off-light path once aspecific TIR threshold is reached.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIGS. 1 a and 1 b are drawings illustrating the operation of a typicalDMD.

FIG. 1 c is a sketch showing how incoming light can get underneath theoff-state mirrors and reflect out into the projection light path,degrading the contrast of the image.

FIG. 2 a is a drawing showing the first embodiment of the presentinvention for a TIR prism assembly used to separate unwanted off-stateand flat-state light from the projection ON light bundle in a single-SLMprojection display system.

FIG. 2 b is a drawing of the TIR prism of FIG. 2 a showing how unwantedlight from OFF surfaces in and around the SLM is separated from the ONlight bundle and directed to an optical heat sink.

FIG. 2 c is a drawing of the TIR prism of FIG. 2 a showing how unwantedlight from FLAT SLM pixels and other flat surfaces are separated fromthe ON light bundle and directed to an optical heat sink.

FIG. 3 is a drawing showing the preferred embodiment of the presentinvention for a TIR prism, consisting of a three element prism attacheddirectly to the SLM package, in place of the typical package window,which has an off-state and/or flat-state TIR prism in close proximity tothe SLM.

FIG. 4 is a drawing showing a third embodiment of the present inventionfor a TIR prism assembly used to separate unwanted off-state andflat-state light from the projection ON light bundle in a multiple-SLMprojection display system. This prism has TIR surfaces to directlyreceive the unwanted light from the multiple SLMs and direct this lightto an optical heat sink.

FIG. 5 is a drawing showing a fourth embodiment of the present inventionfor a low-cost TIR prism, having a single prism element with two TIRsurfaces to separate the unwanted off-state and/or flat-state light fromthe projection light path and route it to an optical heat sink.

FIGS. 6 a and 6 b are drawings illustrating the operation of a DMDhaving asymmetric micromirrors which tilt +10° in the ON state and lessthan 10° in the OFF state.

FIG. 6C is a sketch showing keeping the off mirror in the near-flatposition reduces the size of the large gap between the mirrors andprevents most of the incoming light from getting underneath the mirror.

FIG. 7 is a drawing showing the asymmetric DMD of FIGS. 6 a and 6 bcoupled with the preferred TIR prism of the present invention to providefast separation of unwanted off-state and/or flat-state light from theprojection light path.

FIG. 8 is a drawing illustrating the TIR prism of FIG. 7 attacheddirectly to the asymmetric DMD and showing the critical angle of thefirst TIR surface chosen to switch the unwanted light to an optical heatsink when the mirror tilts past a predetermined threshold.

FIG. 9 is a block diagram for a single-SLM projection system, which usesthe TIR prisms of the present invention to separate the unwanted lightfrom the off-state and/or flat-state from the projected light bundle.

FIG. 10 is a block diagram for a multiple-SLM projection system, whichuses the TIR prisms of the present invention to separate the unwantedlight from the off-state and/or flat-state from the projected lightbundle in each color prism immediately after the unwanted light isreflected from the SLM.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention discloses prism elements having total internal reflective(TIR) surfaces placed in close proximity to the active area of a SLM toseparate the projection light bundle from off-state or flat-state light,or both simultaneously. Embodiments that provide high contrastprojection solutions are disclosed for both single-SLM and multiple-SLMprojection systems. The TIR critical angle is selected to affect eitherthe off-state light or additionally, any portion of flat-state lightreflected from the SLM. These TIR surfaces are placed to immediatelyreflect this unwanted light as it comes off the SLM, thereby preventingthe contamination of light along the projection path, which tends todegrade the system contrast.

FIG. 2 a is a drawing showing the first embodiment of the presentinvention for a TIR prism assembly, which separates unwanted off-stateand flat-state light from the projection ON light bundle in a single SLMprojection display system. This configuration consists of athree-element prism coupled to a single SLM 200. Incoming illumination218 enters through a first prism element 202, which directs light on tothe active surface of SLM 200. This first prism 202 is attached to theupper portion of a second larger prism 204, which has a first TIRsurface 208 and a second TIR surface 210, for separating and routing thevarious types of reflected light, coming off the surface of the SLM, outof the prism assembly without contaminating the desired ON projectionlight. A third prism 206 is attached to the bottom side of prism 204 toprovide an equal optical working distance on the input and output sideof the projection cone for the ON pixel projection light.

In operation, the first prism element 202 directs the incoming light 218on to the surface of the SLM 200. Light reflected from the ON pixels 212passes back through prism 204, striking the first TIR surface 208 at anangle greater than its critical angle, thereby reflecting off the firstTIR surface at an angle less than the critical angle of the second TIRsurface 210, thereby passing through this TIR surface, through the thirdprism 206, along the ON light projection path 220. In a projectionsystem, this light is then coupled through a projection lens on to adisplay screen. The purpose of this third prism is to provide an equaloptical working distance on both the input and output sides of theprojection cone.

Light reflecting from the OFF pixels 216 is shown in FIG. 2 a and inmore detail in FIG. 2 b. Again the incoming light 218 passes through theinput prism 202 where it is directed on to the active area of the SLM200. For the case of the OFF pixels 216, this light is reflected backinto prism 204 where it is further reflected off TIR surface(s) and outof the prism assembly into an optical heat sink 228 (light trap).Depending on the location of a particular pixel, this OFF light may bereflected off the first TIR surface 208 and directly out of the prism222 or it may reflect off both the first 208 and second 210 TIRsurfaces, out of the prism 221 into the optical heat sink 228. Theimportant thing is that light from the OFF pixels 216 is separated fromthe projection light reflecting from the ON pixels 212 and removed fromthe system with minimal contamination of the projected light 220.

Similarly, light reflecting off flat surfaces found in, around, andunder the pixels is also separated from the ON pixel projected light 220and directed into the optical heat sink 228, as shown in FIG. 2 a and inmore detail in FIG. 2 c. Here the incoming light 218 passes through theinput prism 202 where it is directed on to the active area of the SLM200. Light striking surfaces within the SLM package at different angles(near flat) from either the ON or OFF pixels can also be reflected backinto the prism assembly. As with the OFF pixels, this light is furtherreflected off TIR surface(s) and out of the prism assembly into anoptical heat sink 228 (light trap). This FLAT light may be reflected offthe first TIR surface 208 and directly out of the prism 225 or it mayreflect off both the first 208 and second 210 TIR surfaces, one or moretimes, out of the prism 224 into the optical heat sink 228. Again, theimportant thing is that light from the OFF pixels 221,222 and flatsurfaces 224,225 is separated from the ON projection light 220 andremoved from the system.

FIG. 3 is a drawing showing a second preferred embodiment of the presentinvention for a TIR prism, which uses a three element prism attacheddirectly to the SLM package in place of the typical package window. Inthis case, an off-state and/or flat-state TIR surface is located asclose as possible to the SLM in order to separate the unwanted lightfrom the OFF pixels and the flat surfaces around the pixel, from the ONpixel projected light. This embodiment consists of a first prism element310, a second prism element 312, and a prism element third 314, whichare bonded together to provide a first TIR surface 316 and a second TIRsurface 318. The first prism element 310 is attached directly to the SLMpackage 300, completely eliminating the conventional optical windownormally used with SLM devices. Incoming light 320 enters the secondprism element 312 at an angle greater than the critical angle of thesecond TIR surface 318 and is reflected off this TIR surface, throughthe first prism element 310 on to the active surface of a SLM 302. Inthe case of ON-state SLM pixels 304, light is reflected back into theprism assembly at an angle less than the critical angle of both thefirst and second TIR surfaces 316, 318 and therefore passes through theassembly along the projection path 322. In a projection system, thislight is coupled into a projection lens and focused on to a displayscreen.

On the other hand, light reflected from the OFF pixels 306 and flatpixels 308, which may be transitioning from ON to OFF state or the OFFto ON state, as well as flat surfaces on and around the device, arereflected back into the prism assemble at an angle greater than thecritical angle of the first TIR surface 316 and as a result areimmediately reflected out of the assembly. Both this OFF-state light 324and FLAT-state light 326 are directed into an optical heat sink andremoved from the system.

This approach maximizes system contrast and image quality. Theelimination of unwanted light early in the optical path reduces scatterand thermal stresses from optics and mounting hardware around the prismand in the projection path.

The TIR angles may be selected to affect only the off-state light or,additionally, any portion of flat-state and intermediate angles of lightreflecting from the SLM surface. For example, a shallow TIR angle of 25degrees will operate primarily on the higher angled off-state lightrays. A larger TIR angle of 33 degrees will filter additional flat-statelight. Additionally, it is possible to select an optimum index ofrefraction for the TIR elements that, along with the TIR angle, willfurther promote the separation of the projection light from other lightangles.

It is important to note that by attaching the prism assembly directly tothe SLM, dirt, condensation, and other contamination that otherwise mayget between the prism and the SLM window, is largely eliminated. Theelimination of the SLM window also means one less optical surface thatthe ON projection light has to traverse. In addition, componentquantity, size and cost are all reduced. Also, the first optical surfacecan be farther away from the focal point of the SLM active area withoutcompromising the element's thickness and structural integrity. This isoptically preferred, since any blemishes are pushed out of focus.

FIG. 4 is a drawing showing a third embodiment of the present inventionfor a TIR prism assembly where unwanted off-state and flat-state lightis separated from the projection ON light bundle in a multi-chip [two ormore, three shown for red (R), (G), (B)] SLM projection display system.This prism assembly incorporates TIR surfaces 410, 412 as close aspossible to the SLM devices 400, 402 to directly receive the unwantedlight from the SLMs and direct this light to an optical heat sink (notshown). The assembly consists of prism 406, 408 for getting white light416 into the system and getting the projected ON light 420 out of thesystem into a projection lens (not shown) in a display application, acolor splitting/recombining prism assembly 404 for splitting the whitelight 416 into the three R-G-B color beams, and three respective red (R)402, green (G) 400, and blue (B) (not shown but located on the oppositeside of the prism from the red device). The three prism elements haveadditional TIR surfaces 410, 412 for green and red, respectively, (bluenot shown) placed as close to the SLM devices as possible forimmediately separating the unwanted OFF and flat light from the ONprojected light.

In operation of a three SLM system, white light 416 enters prism 406 andat angle greater than the critical angle of TIR surface 414 and isreflected into the color splitting prism assembly 404, where the lightis split into three (R,G,B) beams by means of TIR surfaces and is thenreflected on to the active surface of the three respective SLMs.Reflected light from ON pixels (green light from pixel 418 shown) isthen recombined in the prism assembly 404 and reflected at an angle lessthan the critical angle of the TIR surface 414 back through theinput/output prisms 406/408, along the projection path 420. Output prism408 provides an equal working distance for both the input and outputside of the projection cone.

Light reflected from the OFF pixels 422 (shown for green light only)enters the respective color prisms at an angle greater than the criticalangle of off-light TIR surface and is immediately reflected 424 (shownfor green light only) out of the system into an optical heat sink,effectively separating the unwanted OFF light from the desired ONprojection light 420. Likewise, light reflected from the FLAT surfacesin the device package, enters the respective color prisms at an anglegreater than the critical angle of off-light TIR surface and isimmediately reflected 426 out of the system into an optical heat sink,effectively separating this unwanted FLAT light from the desired ONprojection light 420. These high-brightness multi-chip displayapplications particularly benefit from the high contrast, precision, andvalue added gained by removing the unwanted light as soon as possibleafter being reflected from the surface of the SLM.

FIG. 5 is a drawing showing a fourth embodiment of the present inventionfor a low-cost, single-element TIR prism having two TIR surfaces toseparate the unwanted off-state and/or flat-state light from theprojection light bundle and route it to an optical heat sink. Althoughsomewhat larger is physical size, a single TIR prism element tends to belower cost and is therefore attractive in lower end systems where costmay be more important than size. This configuration consists of thesingle prism, which has a first side 500 with a first TIR surface 502, asecond side 504 with a second TIR surface 506, a third side 508, and aSLM deice 510 mounted in close proximity to the third side 508 of theprism. Optionally, as in the earlier cases, the SLM package can beattached directly to the third side 508 of the prism.

In operation, incoming light 518 enters the through the first side 500of the prism at an angle greater than the critical angle of the secondTIR surface 506 and reflects off this TIR surface on to the active areaof the SLM device 510. Light striking the ON pixels 512 of the SLM isreflected off at an angle greater than the critical angle of the firstTIR surface 502, located on the first side 500 of the prism, andreflects out of the second side 504 of the prism along a projection path520. In a display application, this ON projection light is coupled intoa projection lens. On the other hand, light reflected from the OFFpixels 514 of the SLM strike the first TIR surface 502, located on thefirst side 500, at an angle less than its critical angle and passesthrough the surface, being bent along a first off-light path 522.Similarly, light from flat-state mirrors 516 making the transition fromOn to OFF or OFF to ON and from other flat surfaces in, around, andunder the pixels are reflected on to the second TIR surface 502 at aslightly different angle, but still less than the surface's criticalangle, and also passes through the surface, being bent along a secondflat-light path 524. As in the previous cases, this unwanted off-lightand flat-light is directed into an optical heat sink and discarded.

Alternatively, the two output paths may enable an optical switchfunction. Also, other light paths are possible besides the one discussedin this example, such as light entering the second surface 504 with tworesultant outputs that exit the first surface 500 as spatial or angularseparate bundles.

By optimally using the available surfaces in a single prism element, thedifferent light paths and bundles can be separated without theintroduction of additional elements, surfaces and assembly steps. As aresult, this solution reduces the complexity of design and production,compared to other approaches. Even though the path length in glass maybe larger than other designs, the cost of hardware and assembly issignificantly reduced. Additionally, it is anticipated that bothtelecentric and non-telecentric architectures can take advantage of thisembodiment of the invention to enable further improvements in contrastand light output.

FIGS. 6 a through 6 c are drawings illustrating the operation of a DMDhaving asymmetric micromirrors which tilt typically +10° in the ON stateand less than 10° (typically 0° to −4°) in the OFF state. In a typicalDMD projector, as discussed in FIG. 1, the DMD tilt angles are symmetricabout the projection axis, which in turn is nearly perpendicular to theDMD active mirror array. The DMD tilt angle in both On-state andoff-state conditions is normally equal in magnitude to ½ the projectioncone angle. As an example, for a desired projection f-number of f/2.9,the included projection light is a 20-degree cone and the DMD tilt angleis typically +10 degrees for on-state and −10 degrees for off-state.

Although larger degrees of DMD tilt angle enables increased etendue andlumen output, the problems encountered at the DMD tend to be increasedwhen the tilt angles are increased, including increased bias andswitching voltage, slower switching times, larger required micromirrorgap width, and higher mechanical stress/strain on the torsion hinges.Also, higher tilt angles affect contrast by allowing more light toexpose the region and components under the tilted mirror in theoff-state, as discussed in FIG. 1 c.The illumination angle combined withthe DMD's off-state tilt angle cause much of this unwanted exposure,resulting in light scatter and inevitable loss of contrast.Additionally, any light exposing the underlying structure that is notreflected is absorbed, causing thermal and reliability problems.

Therefore, it is beneficial to minimize the off-state DMD tilt angle,while achieving the optimum on-state tilt angle required for optimalsystem etendue and image quality. The DMD shown in FIGS. 6 a and 6 baccomplish this by providing a mirror that tilts approximately −10° 608in the ON direction (FIG. 6 a), but only 0° to −4° 610 in the OFFdirection (FIG. 6 b). The DMD consists of a substrate 600, a mirrorassembly hinge support post 602, a rotating yoke 604, and a highlyreflective micromirror 606 attached to the yoke. The yoke mirrorassembly rotates from +10° 608 in the ON state to 0° to −4° 610 in theOFF state.

FIG. 6 c illustrates how keeping the off mirror 614 in the near-flatposition reduces the size of the large gap between mirrors and preventsmost of the incoming light 618 from getting underneath the mirror byreflecting it off the device 616 surface as flat/off-light 622,624. Theincoming light 618 that strikes the ON state mirror 612 is reflected offthe mirror along the projection display path 620. By maintaining a largeON-state tilt angle, the system etendue and lumen output is maintainedand by having a near-flat OFF-state tilt angle, less light gets underthe OFF mirror, preventing scattering of unwanted light that mightotherwise get into the projection light bundle and lower the systemcontrast.

FIG. 7 is a drawing showing a fifth embodiment of the present inventionfor a TIR prism, which uses a three prism elements attached directly tothe package of an asymmetric DMD, discussed in FIG. 6, in place of thenormal package window. An off-state and/or flat-state TIR surface ismounted as close as possible to the DMD in order to separate theunwanted light outside the projection cone, coming from the OFF pixelsand the flat surfaces within the device package, from the desired lightwithin the projection cone. In this case, OFF mirrors 703 are at or nearthe flat-state, with the OFF tilt angle being 0 to −4°. This embodimentalso uses three prism elements, a first 704, a second 706, and a third708 element, which are bonded together to provide a first TIR interface728 and a second TIR interface 730. The first prism element 704 ismounted in close proximity or directly to the DMD package 700, placingthe first TIR surface for removing unwanted light as close as possibleto the DMD. Incoming light 710, bounded by the cone boundaries 712/714,enters the second prism element 706 at an angle greater than thecritical angle of the second TIR interface 730 and is reflected off theTIR surface, through the first prism element 704, on to the activesurface of the DMD 702. In the case of DMD pixels that are in an ONstate 701, light is reflected back into the prism assembly at an angleless than the critical angle of both the first and second TIR surfaces728, 730 and therefore passes through the assembly along the ON lightprojection path 716, which is bounded by cone boundaries 718/720. In aprojection system, this light is coupled into a projection lens andfocused on to a display screen.

On the other hand, light reflected from the OFF pixels 703, as well asflat surfaces on and around the device, are reflected back into theprism assemble at an angle greater than the critical angle of the firstTIR surface 728 and immediately reflected out of the assembly. Thisunwanted light 722, bounded by cone boundaries 724/726, is directed intoan optical heat sink and removed from the system, where it is discarded.

In this case, the DMD is constructed with the off-state tilt anglechosen to cause internal reflection of unwanted light within the filterprism. Depending on the prism properties and illumination angle. the DMDoff-state angle is expected to be between 0 and −4 degrees. When the DMDon-state 701 tilt angle is +10 degrees for an f/2.9 projection cone, thetotal included DMD tilt is between 10 and 14 degrees, compared to thatof conventional DMD included tilt angles of 20 degrees. The DMDoff-state 703 tilt angle may be higher or lower, depending on opticaland mechanical requirements, but will always be less in absolute anglethan the DMD on-state angle. This will enable the system to achievebetter thermal and optical performance compared to conventionalsymmetric DMD tilt angles.

FIG. 8 shows a configuration whereby the prism filter element 804 isattached 806 directly to the DMD package 800 to enable early separationof DMD 802 off-state light from the projection light path. It isanticipated that other types of optical elements, such as holographic,grating or lens aperture filters, may be candidates for the necessaryfiltering function of the off-state light. In this example, the TIRcritical angle, θ, required for use in the system is as follows:

for a f/3 projection system →θ=34.25-degrees

for an f/2.5 projection system →θ=33.0-degrees.

FIG. 9 is a block diagram for a single-SLM projection system, which usesthe TIR prism(s) of the embodiments of the present invention to separatethe unwanted light from the off-state and/or flat-state from theprojected light bundle. The system shown in the example is comprised ofa white light source 900 coupled into integrating optics 902, to collectas much of the light as possible, and then through condensing optics 904to bring the light to focus at a spot near the perimeter of a colorfilter wheel 906. Sequential red (R), green (G), blue (B), and optionalwhite (transparent filter element, not shown) light 908 from the colorfilter wheel 906 is then directed into one side of a prism assembly. Theprism assembly in this example consists of a first element 910 bonded toone side of a second element 912, having a first TIR surface 916 at theinterface of the two elements, and a third element 914 bonded to asecond side of the second element 912, having a second TIR surface 918at the interface of these two elements. The window seal surface of a DMDpackage 920 is attached directly to the outside surface of the firstprism element 910 or optionally, a packaged DMD with optical window canbe placed as close as possible to the first prism element surface.Finally, an optical heat sink 934 is located above the first prismelement 910 to collect the unwanted reflected light from the system.

In operation, the sequential color light 908 enters the second prismelement 912 at an angle greater than the critical angle of the secondTIR surface 918 and is reflected off the surface on to the activesurface of the DMD 922. Light from the ON pixels 924 is modulated andreflected at an angle less than the critical angles of the two TIRsurfaces, back through the prism assembly, along an ON projection path926, into a projection lens 928. However, light reflected from the OFFpixels 930, flat pixels 936, and other flat surfaces inside the devicepackage enters the prism assembly at an angle greater than the criticalangle of the first TIR surface 916 and is immediately reflected (OFF932, FLAT 938) out of the prism into the optical heat sink 934. Sincethe separation of the unwanted light from the projection light bundleoccurs immediately as the light enters the prism assembly, theprojection light has minimal contamination and therefore the systemcontrast remains high. Any of the TIR prism embodiments of the presentinvention can be incorporated into this single-DMD projection displaysystem.

FIG. 10 is a block diagram for a multiple (two or more)-DMD projectionsystem, which uses the TIR prisms of the present invention to separatethe unwanted light from the off-state and or flat-state from theprojected light bundle in each color prism immediately after theunwanted light is reflected from the DMD. This example is for a threeDMD high-contrast, high-brightness projection system. The prism assemblyincorporates TIR surfaces 1022, 1024, and 1026 as close as possible tothe DMD devices (red 1016, green 1018, and blue 1020) to directlyreceive the unwanted light from the DMDs and direct this light to anoptical heat sink (not shown). The assembly consists of a white lightsource 1000, which couples light through condensing optics 1002 and offa turning mirror 1004 into an input TIR prism 1006, where the light isinternally reflected into color splitting/recombining prisms 1010, 1012,1014. These color prisms split the light into three continuous,simultaneous red, green, and blue light bundles, which are reflected offTIR surfaces on to respective red 1016, green 1018, and blue 1020 DMDs.Light that is modulated and reflected from the ON pixels of the threeDMDs is reflected back into the prisms where it is recombined andreflected through an output prism 1008, to provide an optimal opticalworking distance, through a projection lens 1034, along a projectionpath 1036, on to a display screen 1038.

On the other hand, light reflected from the OFF pixels and/or from flatsurfaces of the three DMDs enter the respective prisms, at an anglegreater than the critical angle of additional TIR surfaces 1022, 1024,1026 for removing unwanted light, and is immediately reflected out ofthe prisms as unwanted light 1028, 1030, and 1032, away from therespective projection light bundles, into optical heat sinks.

While this invention has been described in the context of fiveembodiments, it will be apparent to those skilled in the art that thepresent invention may be modified in numerous ways and may assumeembodiments other than that specifically set out and described above.Accordingly it is intended by the appended claims to cover allmodifications of the invention that fall within the true spirit andscope of the invention.

1. A prism assembly for use with projection systems, comprising: a prismhaving a first side TIR surface with a critical angle, a second side TIRsurface with a critical angle, and a third side; a display panel mountedin close proximity to said third side of said prism; incomingillumination entering through said first side of said prism, strikingsaid second TIR surface on said second side of said prism at an anglegreater than that of the critical angle of said second TIR surface,reflecting from said second TIR surface to the active surface of saiddisplay panel; reflected light from ON pixels of said display panelpassing through said prism, striking said first TIR surface on saidfirst side of said prism at an angle greater than the critical angle ofsaid first TIR surface, and reflecting out of said prism through saidsecond side of said prism along a projection path; light reflected fromOFF pixels of said SLM striking said first TIR surface on said firstside of said prism at an angle less than the critical angle of said TIRsurface, passing through said first side of said prism into an opticalheat sink; and light reflected from flat-pixels, flat surfaces and edgesin, around, and under said pixels of said SLM striking said first TIRsurface on said first side of said prism at an angle less than thecritical angle of said TIR surface, passing through said first side ofsaid prism into an optical heat sink.
 2. The prism assembly of claim 1,wherein said projection illumination passes through a single prismelement and one display panel package window.
 3. The prism assembly ofclaim 1, said third side of said prism forming a lid of a packageenclosing said display panel.
 4. The prism assembly of claim 1, whereinsaid display panel is a micromirror device.
 5. The prism assembly ofclaim 4, wherein said micromirror device has symmetric micromirrors,which tilt the same number of degrees in both the positive and negativedirection about a diagonal axis.
 6. The prism assembly of claim 4,wherein said micromirror device has micromirrors which tilt x degrees inthe positive direction and less than x degrees in the negativedirection.
 7. A display system comprising: a light source providingillumination light along an illumination light path; a first interfaceallowing said illumination light to pass and reflecting on state lightfrom a display panel; a second interface reflecting said illuminationlight and passing on state light, and a third face passing saidillumination light to said display panel and passing said on state lightfrom said display panel.
 8. The display system of claim 7, said firstinterface allowing off state light to pass.
 9. The display system ofclaim 7, said first interface allowing flat state light to pass.
 10. Thedisplay system of claim 7, said display panel comprising a micromirrordevice.
 11. The display system of claim 7, said display panel comprisinga micromirror device having an on state tilt angle greater than an offstate tilt angle.
 12. The display system of claim 7, comprising a colorsplitting prism between said third face and said display panel, whereinsaid display panel comprises three display panels.
 13. The displaysystem of claim 7, said third face forming a portion of a packageenclosing said display panel.
 14. The display system of claim 7, saidthird face forming a portion of a package enclosing said display panelcomprising a micromirror device.
 15. The display system of claim 7, saidthird face forming a portion of a package enclosing said display panelcomprising a micromirror device having an on state tilt angle greaterthan an off state tilt angle.
 16. An illumination prism comprising: afirst interface for allowing illumination light to pass and reflectingon state light from a display panel; a second interface for reflectingsaid illumination light and passing on state light; and a third face forpassing said illumination light to said display panel and passing saidon state light from said display panel.
 17. The illumination prism ofclaim 16, said first interface allowing off state light from saiddisplay panel to pass.
 18. The illumination prism of claim 16, saidfirst interface allowing flat state light from said display panel topass.
 19. The illumination prism of claim 16, said third face forming aportion of a package enclosing said display panel.
 20. The illuminationprism of claim 16, said third face forming a portion of a packageenclosing said display panel comprising a micromirror device.